1. Field of the Invention
The present invention relates to an image forming apparatus and more specifically to a configuration for detecting a size of a sheet stored in a sheet storage portion.
2. Description of the Related Art
Conventionally, there is known an image forming apparatus such as a copier, a laser beam printer or the like configured to be able to attach a universal cassette, i.e., a sheet storing portion, capable of storing different sizes of sheets. Here, a sheet size detecting portion configured to detect sizes of the stored sheets is required to use the universal cassette.
As such a sheet size detecting portion, Japanese Patent Application Laid-open No. 2007-204249 discloses one using a plurality of switches. The sheet size detecting portion includes the plurality of switches provided in a body of the image forming apparatus, a sheet restricting portion provided in a sheet feeding cassette and movable corresponding to sheet sizes, and a sheet size detecting member moving in conjunction with the sheet restricting portion. The sheet size detecting portion is configured to discriminate the sheet sizes by combinations of ON and OFF, i.e., by switch patterns, of the plurality of switches turned ON and OFF by the sheet size detecting member interlocked with the sheet restricting portion moving to a position corresponding to the sheet size when the sheet feeding cassette is attached to the body of the image forming apparatus.
By the way, in a case of discriminating whether the size of a stored sheet is size A or size B (assumed to be A<B) by using such a mechanism, a point for switching the switch patterns is set between the sizes A and B. Then, if a switch pattern at a position under the switching point is detected, the size of the stored sheet is discriminated to be the size A, and if a switch pattern at a position above the switch pattern is detected, the size of the stored sheet is discriminated to be the size B.
In a case where the sheet sizes are close, i.e., in a case where lengths in the sheet feeding direction of sheets (referred to as a ‘sheet length’ hereinafter) are close, the switching point is set within a very narrow range in such a conventional image forming apparatus. For instance, a difference of sheet lengths is only 4 mm between a 16K size sheet (270 mm in sheet length) and an EXEC size sheet (266 mm in sheet length). In this case, the switching point of the switch pattern is set within the range of 4 mm in discriminating the size only by a position of the sheet restricting portion, i.e., a sheet rear end restricting member.
For instance, there is a case where the switch pattern is set to be switched upstream by 2 mm of a rear end of the EXEC size sheet. In this case, if a user of the image forming apparatus erroneously sets the rear end restricting member by displacing by several mm in setting the EXEC size sheet, the stored sheet is discriminated to be the 16K size sheet.
If size information of the sheet to be used by the user and inputted from a personal computer or the like is different from the sheet size detected by the sheet size detecting unit, conventionally it results in an error in general. Conventionally, while there is known a sheet feeder that is capable of feeding a sheet even if the rear end restricting member is displaced by several mm, the sheet feeder is controlled so as not to feed the sheet even if it is mechanically capable of feeding the sheet if the error occurs.
That is, even though the sheet can be fed mechanically, the sheet cannot be fed if the setting position of the rear end restricting member is displaced by several mm. In the case where the difference of the sheet lengths is thus small, an allowance of the displacement of the rear end restricting member becomes small because the switching point is set within a very narrow range.